The present invention relates to integrated circuits and, more particularly, to forming isolation regions in CMOS (complementary metal oxide semiconductor) image sensors.
Digital cameras are often provided with digital image sensors such as CMOS image sensors. Digital cameras may be stand-alone devices or may be included in electronic devices such as cellular telephones or computers. A typical CMOS image sensor has an image sensor pixel array containing thousands or millions of pixels. Each pixel includes a photosensitive element such as a photodiode formed in a substrate. Isolation regions may be formed in the substrate between photodiodes to reduce crosstalk between photodiodes.
To improve image quality, it is often desirable to increase the number and density of pixels on an image sensor. As pixel density increases, pixels necessarily are pushed closer and closer together, increasing the likelihood of cross-talk. Isolation regions help alleviate cross-talk and allow the photodiodes to have a greater full well capacity and therefore an improved image quality.
Conventional methods for forming isolation regions include ion implantation. However, implanted ions are difficult to precisely control and often diffuse laterally, making it impossible to produce an abrupt junction. Consequently, full well capacity must be sacrificed in order to provide sufficient isolation between photodiodes. Alternatively, deep trench isolation methods may be used in which a liner oxide is grown in an isolation trench. However, this method introduces defects due to lattice mismatch, thus resulting in higher dark current. Dark current refers to the current that is spontaneously generated even when a pixel is not exposed to light. The increase in dark current resulting from a liner oxide in an isolation trench can result in undesirable noise or hot pixels in the image sensor.
It would therefore be desirable to be able to provide improved methods for forming isolation regions in image sensors.